The borehole of producing oil and gas wells is typically lined from top to bottom with steel casing anchored by a sheath of cement that is securely and circumferentially bonded to both the casing and the wall of the wellbore. Offshore wells are sometimes abandoned with no equipment projecting above the water surface. Oftentimes wells are drilled at extreme depths with the drilled wellbore deviating sustantially from the vertical. There is, therefore, the need for a reliable method of locating the wellhead of such abandoned offshore wells or for locating the bottom of a deviated well, especially in the case of a well blowout when a relief well is to be drilled to intersect the deviated well at a point above or near the blowout. Other similar situations may arise when the exact location of the wellhead or well bottom is needed.
One of the methods that has been used in such well location efforts is by searching with a magnetometer for the magnetic anomaly created by the well casing. The natural magnetization of a well casing due to the earth's magnetic field produces an anomaly in the total magnetic field which may be detected with a sensitive magnetometer at distances up to a few hundred feet. The magnitude of the anomaly is proportional to the end steel area of the casing, the vertical component of the earth's field, the effective permeability, and is inversely proportional to the square of the distance from the wellhead or well bottom. However, the vertical component of the earth's magnetic field goes to zero at the magnetic equator. Thus, the reduced anomaly over about twenty percent of the earth's surface is difficult to impossible to detect.